9 8 



F. G. HALL. 



high in carbon dioxide being avoided by the fishes. From the 

 experiments described in the present paper, it may be assumed 

 that such distribution is due, at least in part, to a more or less 

 automatic mechanism which operates so as to control vertical 

 distribution by altering specific gravity. Both these processes 

 are ordinarily adaptive and may operate simultaneously. 



The results of these experiments show that when fishes are 

 subjected to increased amounts of dissolved carbon dioxide in 

 water, the carbon dioxide in the swimbladder increases and is 

 roughly proportional to that in the water. Fig. 4 illustrates 

 this point. Carbon dioxide produces an alteration in the specific 

 gravity of the fish, causing it to rise and escape from regions 

 which might be unfavorable for its existence. 



FIG. 6. A diagram to show the carbon dioxide gradients with depth in Lake 

 Mendota for the month of August from 1906-1920. The data were obtained from 

 Birge and Juday. The minus sign indicates alkaline water and the plus sign acid. 



DIFFUSION AND SECRETION. 



A fluid gives off gas to or takes gas from any other medium 

 with which it is in contact according to the relative pressure 

 of the gases. Dalton's Law of Partial Pressures may be stated 

 as follows: in a mixture of gases each gas exerts the same pres- 

 sure as it would exert if it were alone present in the volume 

 occupied by the mixture. The pressure of each gas is called 

 its partial pressure. 



It is believed by most physiologists that the inter-change of 

 gases during respiration is explained by the laws of physical 

 diffusion. The interchange of gases in the lungs has raised a 

 problem that has inspired considerable study. While many 

 physiologists have accepted the view that during the interchange 

 of gases in the lungs, the membranes between the blood and the 



